Add wishlists_count to Spree::user_class and products_count to wishlists

[channainfo]

I have a scenario in the mobile app to show how many wishlists a user has in his profile along with how many products a specific wishlist has.

Is this something you would accept as a PR?

[damianlegawiec]

@channainfo wishlists were added to spree core in v4.4 - so you can freely submit a PR to spree/spree